1. Field of the Invention
Aspects of the present invention relate to an image forming apparatus, and more particularly to a color image forming apparatus that is provided with plural photosensitive bodies on which visible images are formed, and forms a color visible image by overlapping the visible images of respectively different colors formed on the respective photosensitive bodies.
2. Description of the Related Art
An image forming apparatus is an apparatus that prints a black and white image or a color image on a printing medium (e.g., paper) according to an image signal. Examples of an image forming apparatus include a laser printer, an ink-jet printer, a copying machine, a multi-function printer, a fax machine, etc. An image forming apparatus is classified as an electrophotographic type or an ink-jet type. In the electrophotographic image forming apparatus, a beam is scanned onto a photosensitive body to form an electrostatic latent image and a developer is adhered to the electrostatic latent image to transfer the same onto a printing medium. In the ink-jet image forming apparatus, a liquid type ink is ejected onto a surface of a printing medium according to an image signal.
Furthermore, the electrophotographic image forming apparatus is configured such that a surface of a photosensitive body is charged with a predetermined electric potential, a beam is scanned onto the photosensitive body to form an electrostatic latent image due to an electric potential difference, and a developer is adhered to the electrostatic latent image to form a visible image. The visible image formed on the photosensitive body is transferred onto the printing medium, and is fixed to the surface of the printing medium by applying heat and pressure to the printing medium.
Such an electrophotographic image forming apparatus is classified as a mono image forming apparatus that prints a black and white image, or a color image forming apparatus that prints a color image. The mono image forming apparatus uses only a black developer and thus requires only a black developing device. However, the color image forming apparatus requires a plurality of developing devices, typically, of four colors (e.g., magenta, cyan, yellow and black) to form a color image.
There are two types of currently well-known electrophotographic color image forming apparatuses. Specifically, a multi-path type color image forming apparatus is provided with one photosensitive body, and a single-path type color image forming apparatus is provided with a plurality of photosensitive bodies corresponding to the respective developing devices.
The multi-path type color image forming apparatus is configured such that one developing device operates while one photosensitive body rotates once in order to form a visible image of one color on the photosensitive body. The visible image formed on the photosensitive body is then transferred before another developing device operates. The formation of the visible image of one color is repeated by the remaining developing devices. Therefore, the exposure process, the developing process and the transfer process are repeated four times in order to obtain a color image. In particular, the visible images of the respective colors are overlapped in order on the intermediate transfer device and transferred together onto the printing medium.
The single-path type color image forming apparatus is configured such that the visible images of the respective colors are formed in order on the respective photosensitive bodies provided corresponding to the respective developing devices at a slight time difference. Then, the visible images formed on the respective photosensitive bodies are transferred in order onto the intermediate transfer device or the printing medium at a slight time difference and overlapped. Therefore, the single-path type color image forming apparatus has an advantage that the image forming time is shorter than the multi-path type color image forming apparatus.
One example of the single-path type color image forming apparatus is disclosed in Japanese Patent Laid-open Publication No. 11-52651 (Feb. 26, 1999). The disclosed conventional single-path type color image forming apparatus is configured such that the visible images of the respective colors formed by four developing devices are transferred in order onto the printing medium fed by a belt, and overlapped on the printing medium. The developing devices are respectively provided with photosensitive bodies on which the visible images are formed. The photosensitive bodies are respectively provided with driving gears to transmit power. The driving gears of the respective photosensitive bodies are connected to one photosensitive body driving motor through gear trains including a plurality of gears.
However, in the above conventional color image forming apparatus, the power transmission mechanism is complicated because the photosensitive bodies are connected to the photosensitive body driving motor through a plurality of gears. Moreover, the gears should have small sizes due to limits of the mounting space. Thus, as modules of the gears are small, the loads applied to teeth of the gears are large, so that the teeth of the gears may be easily deformed or broken. Furthermore, the deformation or damage to the gears causes a printing error (such as jittering or banding) and a deterioration in printing quality.
Even more, the large number of power transmission gears causes an assembling error of the gears and, consequently, a rotational imbalance of the respective photosensitive bodies. Accordingly, a phase difference between the visible images transferred onto the printing medium becomes large, and a color registration error very possibly occurs.